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Motor current to voltage converter

  1. Dec 24, 2008 #1
    Hi all..

    I'm designing a Safe deposit box..
    It use motor to open and close the box..
    but i want to know if the door is blocked when closing/opening..

    I realize that the current needed for the motor will increase if it's blocked..

    I'm using ATmega ADC, but it need voltage input..
    so i need a current-voltage converter to know if it's blocked or not..

    can someone provide me with circuit and explanation?


  2. jcsd
  3. Dec 24, 2008 #2


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    Current shunt? (There are high-precision < 1 Ohm resistors sold for this purpose)
  4. Dec 24, 2008 #3
    hi.. can u help me by providing the schematic (with the motor)..

    I'm still newbie in electronics..

  5. Dec 24, 2008 #4


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    You just need to put the shunt resistor at the tail end of your h-bridge. That's presuming that you're using a DC motor here, by the way.

    The shunt resistor will have a small voltage generated because all of the current going through the motor will go through it before going to ground (V=I*R_shunt). You just measure this voltage (using the ADC, and maybe an amplifier--remember, it's not a very high voltage when you only have 50 mOhm) and you can calculate the current (or just set a threshold ADC value for a short condition, without having to do any of this math).
  6. Dec 24, 2008 #5


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    Staff: Mentor

    You need to do the schematic yourself -- it's *your* project. You should be able to do it with he advice that MATLAB has provided so far, plus some additional research on your own. We do not do your homework or your project schematics for you here on the PF.
  7. Dec 24, 2008 #6


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    Just wanted to throw out another posibility if this is not a school project; although using a motor loading circuit that you are designing will work fine. One could use some sort of proximity swith to sense the door not being fully closed.

    On second thought, your design might be better in that you could also make it reverse the motor so that you could remove your finger should it get in there. :surprised
  8. Dec 24, 2008 #7
    Just a curious side note, but is this how garage doors work when they sense someone (or something) stuck under it? I could see the shunt trick being a quick and easy way to sense something blocking the path.
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